1. Field of the Invention
This invention relates to a method for preparing a foam of chlorinated polymer of vinyl chloride.
2. Description of the Prior Art
Foams of chlorinated polymers of vinyl chloride have good heat resistance when compared to foams of the non-chlorinated polymers of vinyl chloride. More particularly, foams of chlorinated vinyl chloride polymers have a small coefficient of contraction at high temperatures and can withstand the high temperatures, and therefore can be used for applications in which the foam is contacted with hot water or steam pipes for heat insulation. Furthermore, the foam of the chlorinated polymer is superior in non-inflammability and is suitable for use as construction materials. Moreover, the foam of the chlorinated polymer is superior in mechanical strength and in electrical insulation properties. The foam of the chlorinated polymer also has excellent chemical resistance, and can be used in apparatus for the chemical industry. It is expected, therefore, that the foam of the chlorinated chloride polymer will be used in wide variety of applications.
There is, however, no known satisfactory process for manufacturing a foamed article of a chlorinated polymer of vinyl chloride. For example, U.S. Pat. No. 3,366,580, discloses a process wherein a chlorinated polymer of vinyl chloride is impregnated with halogenated hydrocarbons as the foaming agent, and is then heated to obtain a foamed product.
The process, however, has many drawbacks. Firstly, the resulting product is inferior in heat resistance. The reason for this is believed to be that the halogenated hydrocarbons have an excellent affinity for the chlorinated vinyl chloride polymer and a large amount of the halogenated hydrocarbons is liable to be retained in the foamed product. The halogenated hydrocarbons remaining in the foamed product act as plasticizers when exposed to high temperatures. The foamed product, therefore, is inferior in heat resistance. Since a chlorinated polymer of vinyl chloride is superior to a non-chlorinated polymer of vinyl chloride with respect to heat resistance, and since this is one of the main characteristics of the chlorinated polymer of vinyl chloride, it is to be understood that it is a significant drawback of the chlorinated polymer of vinyl chloride that the heat resistance is deteriorated after the chlorinated polymer has been foamed.
Secondly, the halogenated hydrocarbons have, in general, a high toxicity and pollute the working environment. Furthermore, the halogenated hydrocarbons, when heated, decompose to generate a halogen and a hydrogen halide which also pollute the working environment or attack the apparatus used therein.
Thirdly, the process gives a foamed article that has inferior heat stability and is liable to discolor when exposed to high temperatures.
The above-mentioned drawbacks are all due to the fact that halogenated hydrocarbons are used as the foaming agent for the chlorinated polymer of vinyl chloride.
The inventors tried to improve or avoid these drawbacks and found that a resin foam could be obtained having fine closed cells, when an alcohol having from 1 to 5carbon atoms was used for the foaming agent. The inventors also determined that alcohols having 1 to 5 carbon atoms are superior in foaming property to halogenated hydrocarbons, and that the alcohols are advantageous in that they do not deteriorate the heat-resistance and heat stability of the chlorinated polymer foam and have, in general, a low toxicity.